Field of the Invention
The present invention relates to a refrigeration and/or liquefaction device and to a corresponding method.
The invention relates more specifically to a device for the refrigeration and/or liquefaction of a working gas containing helium or consisting of pure helium, the device comprising a working circuit in the form of a loop for the working gas.
Related Art
The invention relates notably to helium refrigerators/liquefiers generating very low temperatures (for example 4.5K in the case of helium) with a view to continuously cooling users such as superconducting cables or components of a plasma generation device (“TOKAMAK”). What is meant by a refrigeration/liquefaction device is notably the very low-temperature (cryogenic temperature) refrigeration devices and/or liquefaction devices that cool, and where appropriate liquefy, a gas with a low molar mass such as helium.
When the point of use is cooled down, which means to say when the point of use needs to be brought down from a relatively high starting temperature (for example 300K or above) to a determined low nominal operating temperature (for example around 80K). The refrigeration/liquefaction device is generally ill-suited to such cooling.
What happens, when heavy components (such as superconducting magnets for example) are cooled from ambient temperature down to 80K over a lengthy period (over a few tens of days), relatively hot and cold streams of helium (feed toward the point of use and return from the point of use) pass countercurrentwise through common exchangers. For the device to operate correctly though, it is necessary to limit the difference in temperature between these streams of helium (for example to a maximum difference of between 40K and 50K).
To do so, the device comprises an auxiliary pre-cooling system which supplies negative calories during this cooling-down.
As illustrated notably in the article (“Solutions for liquid nitrogen pre-cooling in helium refrigeration cycles” by U. Wagner of CERN-2000), the pre-cooling system generally comprises a volume of liquid nitrogen (at constant temperature, for example 80K) which supplies negative calories to the working gas via at least one heat exchanger.
These known pre-cooling systems do, however, have constraints or disadvantages.
Thus, it is necessary to mix helium at 80K with hotter helium (at ambient temperature or the temperature at which it returns from the point of use that is to be cooled).
In order to limit the consumption of liquid nitrogen it is moreover necessary to recover the negative calories from the helium returning from the point of use that is to be cooled as the point of use is gradually cooled. These constraints on temperature difference and on performance require heat exchanger technologies that differ according to the various operating configurations (cooling-down, normal operation).